When using injection devices as defined above, a desired dose is normally set by means of a dose setting mechanism. When the dose has been set, an injection needle is positioned at a desired injection position, and the set dose is injected by means of an injection mechanism, normally including a manually operable injection button and a piston rod cooperating with a piston of a cartridge containing the drug to be injected.
If it is possible for the piston rod to move in a distal direction during setting of a dose, there is a risk that liquid drug is accidentally spilled during dose setting. Furthermore, if it is possible for the piston rod to move in a proximal direction during dose setting, the pressure applied to the piston from the piston rod may decrease, possibly even forming a gap between the piston and the piston rod, and thereby there is a risk that blood is sucked into the cartridge when the injection needle of the device is subsequently inserted at the injection site. Furthermore, if it is possible for the dose setting mechanism to move during injection of an initially set dose, the set dose may accidentally be increased or decreased during the injection, and the actually injected dose may thereby be unknown. This is very undesirable. It is therefore desirable to be able to prevent such undesired movements of the piston rod and the dose setting mechanism, respectively.
Attempts have been made to solve the problem described above in injection devices of the kind having a cylindrical scale drum with a helical spiral of numbers written thereon. When a dose is set in such an injection device the cylindrical scale drum is dialled outwards, and a number corresponding to the set dose will show in a window on the injection device. US 2004/0059299 discloses an injection device of this kind. The injection device is provided with a dose setting element which, during dose setting, is coupled rotationally as well as axially to the scale drum. During injection the dose setting member is prevented from rotating. At the same time the dose setting element is coupled to a dose rod which can be rotated along a piston rod via a gearwheel providing a gearing corresponding to the pitch of the scale drum. Since the dose setting element and the scale drum are axially coupled during dose setting, the dose rod is rotated in a controlled manner, thereby preventing the piston rod from moving during dose setting. Since the dose setting element is prevented from rotating during injection, the dose rod is pushed back without performing a rotational movement, thereby causing an advancing movement of the piston rod.
EP 1 304 129 discloses a medication dispensing device including a lockout mechanism that prevents the dial from being depressed during dosing. The apparatus includes a drive assembly mounted to the housing and manually advanceable in the housing between a dose setting position and an injection position for manually moving the drive stem to drive the piston within a container. The drive assembly is locked from movement with respect to the housing along the axis of ejection while in the dose setting position. A disengaging device is secured to at least one of the drive assembly and the housing to unlock the drive assembly from the housing to enable the drive assembly to be axially advanceable with respect to the housing to move the drive assembly from the dose setting position to the injection position.
The injection device disclosed in EP 1 304 129 is also of the kind having a cylindrical scale drum, and the mechanism described above is connected to the scale drum.
It is a disadvantage that the mechanism which prevents undesired movements of the piston rod during dose setting is connected to the scale drum, since the mechanism therefore can not readily be applied to injection devices which are not provided with a cylindrical scale drum. Furthermore, it is sometimes desirable to design an injection device without such a cylindrical scale drum, e.g. in order to use the space occupied by a scale drum for other purposes, e.g. additional electronics for controlling the injection device, or simply because a different counter mechanism is desired. However, it is still desirable to be able to overcome the problems outlined above in such an injection device.